1. Field of the Invention
This invention relates to a method for determining fluid perfusion of tissue by the use of microwave energy.
2. Description of the Prior Art
Researchers in medical science have known that the rate of blood flow within a tissue can be measured by a process of heating a device, such as a probe, which is in contact with the tissue being examined, and then recording the temperature changes by a thermocouple positioned in or near the probe. The recorded temperature changes are representative of the blood flow in the tissue. The heated device and thermocouple effectively act as a flowmeter for determining the blood flow as a function of the rate at which heat is carried away from the tissue.
Heated probes and thermocouples used for the determination of blood flow were first introduced by F. A. Gibbs in 1933 for the purpose of measuring flow in blood vessels. Gibbs' experiment is described in Proc. Soc. Exptl. Biol. Med. 31; 141-147, 1933, entitled, "A thermoelectric blood flow recorder in the form of a needle." Heated probes and thermocouples were later used as flowmeters by C. F. Schmidt and J. C. Pierson for measuring blood flow in solid organs. Schmidt's and Pierson's efforts are described in the Am. J. Physiol., 108; 241, 1934, entitled, "The intrinsic regulation of the blood flow of medulla oblongata." Further investigation by J. Grayson and his colleagues described in Nature 215: 767-768, 1967, entitled, "Thermal Conductivity of Normal and Infarcted Heart Muscle," demonstrated that a heated probe with a thermocouple could be used in accordance with a certain relation, known as Carslaw's equation, to measure the thermal conductivity (k) of any solid, semisolid, or liquid in which the heated probe and thermocouple were inserted. Carslaw's equation is discussed in detail in the Journal of Applied Physiology, Vol 30, No. 2, February 1971, in an article entitled, "Internal Calorimetry Assessment of Myocardial Blood Flow and Heat Production."
A heated coil about a themistor may also by used as an effective flowmeter as described in a Technical Note entitled, "Thermal Transcutaneous Flowmeter," by D. C. Harding, et al., published in Med. & biol. Engng., Vol. 5, 623-626, Pergamon Press, 1967, Printed in Great Britain.
"Heated" thermocouples or thermistors used in flowmeters, function to provide heat essentially by conduction to the tissue in immediate contact with the heating device, and measure the temperature of that tissue. Determination of fluid (blood) perfusion heretofore was limited by the heating of tissue essentially in contact with a heated device.